I am reading some quantities here that remind me of a severely DIY overstuffed quilt here that I find unpleasant. It is too much to compress, and too much to fight with in the hammock. (That said with due respect for those who willingly share twin beds with others.)
I'd suggest you do a trial with carefully measured quantities of down and a sample chamber, such as a pillow case.
You shouldn't start sewing without tests with your thread and fabric for tensions etc. No different with down of unknown quantity.